Gas-discharge tube for high-current pulsed operation

ABSTRACT

A gas-discharge tube capable of sustaining a cold-cathode arc enables the switching of high-current pulses with a duration of one microsecond at a rate in excess of 100 KH. The operative surface of the cathode bounds at least one cavity in the cathode such that for a majority of points on the surface the outward normal to the surface intersects the surface at a distance not exceeding 5 millimeters and preferably not exceeding 2 millimeters. The cathode surface may be the surface of an annular slit and the cathode material may be copper.

United States Patent [191 Holmes et al.

GAS-DISCHARGE TUBE FOR HIGH-CURRENT PULSED OPERATION Inventors: AndrewJames Timothy Holmes;

John Ritchie Cozens, both of London, England National ResearchDevelopment Corporation, London, England Filed: Aug. 4, 1972 Appl. No.:278,024

Assignee:

Foreign Application Priority Data Aug. 13, 1971 Great Britain 38100/71US. Cl.. 313/217, 313/220, 31'3/247 Int. Cl H01j 61/06, H01j-6l/30 Fieldof Search 313/209; 210, 217', 218,

References Cited UNITED STATES PATENTS 1 1947 Chevigny 313/247 x 113,835,343 [451 Sept. 10, 1974 2,540,019 l/l95l Warnecke 3l3/247 X3,670,201 6/1972 Veron 313/217 X Primary Examiner-Palmer C. DemeoAttorney, Agent, or Firm-Cushman, Darby & Cushman v [5 7] ABSTRACT Agas-discharge tube capable of sustaining a coldcathode arc enables theswitching of high-current pulses with a duration of one microsecond at arate in excess of 100 KB. The operative surface of the cath- 5 Claims, 4Drawing Figures GAS-DISCHARGE TUBE FOR HIGH-CURRENT PULSED OPERATION Thepresent invention provides a form of gasdischarge tube suitable for usein the generation of short current pulses at high repetition rate by therepeated striking and extinction of acold-cathode arc. This class of arcis obtainable when using as cathode a metal whose thermionic emissioncapability at and below its boiling-point is insufficient to provide thecurrent density observed in the arc. The are is localised at a verysmall area of the cathode surface, the position of this area varyingwith time, and immediately above the surface at the arc site is a densecloud of vapour of the cathode metal confined at high pressure bymomentum transfer from incident positive ions.

According to the invention there is provided a gasdischarge tube havingan anode and an unheated metal cathode between which a cold-cathode arcmay be struck by the application of a suitable voltage between them,that part of the surface of the cathode which is exposed to thegas-filling of the tube bounding at least one cavity in the cathode suchthat for at least a majority of points on said part of the surface theoutward normal to the surface intersects said part of the surface at adistance not exceeding 5 millimetres and preferably not exceeding 2millimetres.

Preferably the cathode forms part of the envelope of the tube.

It would appear that the merit of the form of cathode structure used ina tube according to the invention resides in enabling the anode voltage,following extinction of the arc, to be raised rapidly to its originallevel without premature restriking of the arc. It is thought that theability of the discharge gap of a cold-cathode arc device to recover inthis way is dependent on rapid dispersal of the cloud of cathode-metalvapour formed in the arc and that this is achieved in the present caseby the provision of a condensation surface forming part of the cathodewithin close range of the majority of possible arc sites.

One arrangement in accordance with the invention will now be describedby way of example with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic sectional view of the basic construction of adischarge tube embodying the invention;

FIG. 2 is an outline circuit diagram of a relaxation oscillatorillustrating the use of the tube in controlling the delivery of currentpulses to a load; and

FIGS. 3a and 3b are diagrams illustrating voltage and current waveformsfor the tube.

Referring to FIG. 1, the tube comprises a sealed envelope filled with agas at a pressure about 0.5 Torr, the envelope comprising co-axialceramic tubes 1 and 2 each 25 mm in length and diameter and having awall thickness of 3 mm. The nature of the gas filling is not critical,but it is convenient to use hydrogen since this lends itself readily topressure stabilisation using well known replenisher techniques. Theremote ends of the tubes 1 and 2 are respectively sealed by metal plates3 and 4 and the open ends are spaced axially by a gap 7 of width 1 mm,the tubes being sealed adjacent these ends to an external copper ring 5.The ring 5 has a circumferential slit 6 of depth 5 mm extending from itsinner face, the slit 6 having the same width as and being aligned withthe gap 7 between the ceramic tubes 1 and 2. The tube is operated withthe ring 5 as cathode and one of the plates 3 and 4 as anode.

Referring to FIG. 2, the tube 8 is connected in series with an inductor9 across a high voltage d.c. source 10, that one of the plates 3 and 4not serving as anode being left floating. Across the tube 8 areconnected in series a load 11, an inductor l2 anda capacitor 13.

The operation of the circuit for each cycle is as follows, starting fromthe point immediately after the passage of a current pulse through theload 11. At this point the voltage across the capacitor 13 is relativelylow and the capacitor 13 charges from the source 10 via the inductors 9and I2 and the load 11. The inductor 9 is arranged to have a very muchlarger impedance than either the inductor 12 or the load 11 so that thetime constant for charging the capacitor 13 is primarily determined bythe value of the inductor 9 and the capacitor l3.

Depending on the initial value of the capacitor voltage, a residuallow-current glow discharge may exist in the tube 8, but in any eventsuch a discharge will form as the capacitor 13 charges, this dischargebeing maintained by current drawn from the source 10.

As the capacitor voltage is further increased the tube current alsoincreases in the abnormal glow regime until at about 2,000 volts thereis an abrupt transition to an arc regime capable of supporting a highcurrent with only a small voltage drop. The required current cannot bederived from the source 10 because of the presence of the inductor 9 andhence the capacitor 13 begins to discharge through the tube 8, the load11 and the inductor 12. In consequence a large pulse of current passesthrough the load 11, the duration of this pulse being determined by thetime constant of the capacitor l3 and the inductor 12.

FIG. 3a shows the variation of the tube voltage with time, rising to thearc transition level v, and abruptly falling to the arc maintenancelevel V The corresponding change in current is as shown in FIG. 3b. Asthe current pulse decays to a low value the arc ceases to beself-sustaining and the discharge may extinguish completely or mayrevert to a low current glow maintained by current drawn from the source10. Dependent on the extinction condition for the arc a voltageindicated by V in FIG. 3a will remain on capacitor 13.

Capacitor 13 will then recharge and the process will be repeatedcyclically with a periodic time T and a pulse length t, as indicated inFIGS. 3a and 3b provided that T is made several times as large as t. Theamplitude of the current pulse may be controlled according to the valueof the capacitor 13 so that a suitable combination of pulsecharacteristic and periodic time may be chosen to determine the meanpower delivered to the lead 11. A

While a series resonant arrangement is shown in FIG. 2 other knownarrangements (e.g., a parallel resonant arrangement) would be equallyeffective.

As has been described, the instant of initiation of the arc isdetermined by the charging of capacitor 13 to the transition voltage VIn order to obtain great precision in the timing of the initiation ofthe are it is possible in a modification of the arrangement describedabove, to operate as a trigger electrode that one of the plates 3 and 4not serving as anode. In this mode of operation, at a time whencapacitor 13 is approaching the transition voltage V a voltage pulsesufficient to ensure the striking of an arc is applied to the triggerelectrode. The discharge will then immediately transfer to the main gap.Alternatively the trigger pulse may be injected into the supply at apoint between the source and the main anode.

An oscillatory circuit as described above has been operated with a pulseduration of less than l microsecond at a repetition rate in excess ofI00 KHz. The switching rate of the tube istherefore superior to that ofa thyratron or thyristor.

In an alternative arrangement using'a non-oscillatory circuitsingle-shot operation is obtained by applying a standing voltage whichis insufficient to cause an arc and superimposing a trigger voltage whenthe arc is to be initiated.

We claim:

1. A gas-discharge tube comprising: a seated envelope having a gasfilling; an anode exposed to the gas filling; and an unheated cathodehaving only a part of its surface exposed to the gas filling, saidcathode beingof a metal such that a cold-cathode arc' may be struckbetween'the anode and the cathode by the application of a suitablevoltage between them, said only part of the surface of the cathode whichis exposed to the gas filling consisting of at least one cavity in thecathode such that for at least a majority of points on said part of thesurface the outward normal to the surface intersects said part of thesurface at a distance not exceeding 5 millimeters.

2. A gas-discharge tube according to claim 1 in which for at least amajority of points on said part of the surface the outward normal to thesurface intersects said part of the surface at a distance not exceedingtwo millimeters.

3. A gas-discharge tube according to claim 1 in which said part of thesurface of the cathode is the surface of an annular slit. v

4. A gas-discharge tube accordingto claim 1 in which the cathodematerial is copper.

S. A gas-discharge tube according to claim 1 in which the cathode formspart of the envelope of the tube.

1. A gas-discharge tube comprising: a sealed envelope having a gasfilling; an anode exposed to the gas filling; and an unheated cathodehaving only a part of its surface exposed to the gas filling, saidcathode being of a metal such that a cold-cathode arc may be struckbetween the anode and the cathode by the application of a suitablevoltage between them, said only part of the surface of the cathode whichis exposed to the gas filling consisting of at least one cavity in thecathode such that for at least a majority of points on said part of thesurface the outward normal to the surface intersects said part of thesurface at a distance not exceeding 5 millimeters.
 2. A gas-dischargetube according to claim 1 in which for at least a majority of points onsaid part of the surface the outward normal to the surface intersectssaid part of the surface at a distance not exceeding two millimeters. 3.A gas-discharge tube according to claim 1 in which said part of thesurface of the cathode is the surface of an annular slit.
 4. Agas-discharge tube according to claim 1 in which the cathode material iscopper.
 5. A gas-discharge tube according to claim 1 in which thecathode forms part of the envelope of the tube.